This invention pertains to the art of electrical control circuits and more particularly to novel control circuitry for obtaining wide pulse range periodic reversal of current.
The invention is particularly applicable for use as a pulser of direct current or as a periodic reverse unit in the electrolytic plating industry. However, it will be appreciated by those skilled in the art that the invention could be readily adapted to use in other environments as, for example, where similar circuitry is employed to generate a wide pulse range with high current.
The electrolytic plating of a cathode is effected by the deposit of ions electrically drawn from an ambient bath and, in particular, from a zone of the bath adjacent the cathode which is typically referred to as the cathode diffusion layer. During plating, the cathode diffusion layer is depleted of ions. After plating, ions rediffuse into the depleted diffusion layer to the ion concentration level of the ambient bath. When the rate of deposition of ions exceeds the rate of diffusion of ions into the cathode diffusion layer, the current density of the system exceeds the deposition limiting current densities and, a generally undesirable deposit reults. It is common practice to exceed the limiting current density with a plating current. To avoid the effects of exceeding the limiting current density, the plating industry has generally adopted pulse-plating arrangements wherein the time between pulses of plating current the cathodic film is given ample time to rebuild itself. In this regard, greater overall efficiency in the deposit rate results when shorter pulses of very high current are used.
Periodic reversal of current in electrolytic plating has been used in the industry to produce a smoother, better plated surface. During reverse current flow, undesired particles which may have attached themselves to the plating surface may be thrown off, and gross irregularities in the electrodeposits, such as trees or nodules, which have risen above the level of the cathode surface are dissolved and leveled at least in part. It is an inherent characteristic of electroplating that trees rising above the cathode surface grow in electrodeposit rate faster than the surrounding smooth surface. Similarly, during reversal of current, the trees dissolve faster. See generally Lowenheim, Modern Electroplating, 3rd Ed. John Wiley & Sons, Inc. (1974, pgs. 1-43).
Periodic reverse current has been achieved in the past by either mechanical switch means employing relays and mechanical switches or by solid state switching means employing silicon controlled rectifiers or transistors. The mechanical switching means have the primary disadvantages and limitation of slow switching rates. The solid state switching means have the primary disadvantage and limitation of low currents.
The present invention overcomes the above-noted limitations in the prior art and thus provides an improvement in the generation of wide pulse ranges and high currents and in extending their useful applications.